This application claims the benefit under 35 U.S.C. §119(a) of Japanese Patent Application No. 2004-093258, filed in the Japanese Intellectual Property Office on Mar. 26, 2004, and of Korean Patent Application No. 10-2004-0075971, filed in the Korean Intellectual Property Office on Sep. 22, 2004, the entire disclosures of each of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a laser scanning unit. More particularly, the present invention relates to a laser scanning unit that is applied to a tandem color image forming system or the like, and which deflects a plurality of laser beams using one light deflecting means, separates optical paths of the deflected laser beams, and radiates the separated laser beams on a plurality of exposed media.
2. Description of the Related Art
Laser scanning units are typically used in image forming systems, such as printers or digital copiers, to scan laser beams deflected by light deflecting means onto the image forming optical systems using a singlet plastic fθ lens.
Such a laser scanning unit, which can make incident light parallel in a main scanning direction to easily dispose a light source in front of light deflecting means, is disclosed in Japanese Patent Publication No. 2621838 (see FIG. 1), the entire disclosure of which is hereby incorporated by reference.
The laser scanning unit is constructed such that light radiated from a semiconductor laser is collimated into a parallel beam by a collimating lens, and the parallel beam is focused only in a sub-scanning direction, which is a rotational direction, by a cylindrical lens. The focused parallel beam is then imaged on a position at a rotating polygonal deflector or a polygon mirror used as a light deflecting means, and the light is scanned at a constant speed by a singlet scanning lens in an axial direction of a photosensitive drum.
Further, a laser scanning unit, which can reduce a magnifying power necessary for a singlet plastic fθ lens by radiating focused light from a light source and thereby make the singlet plastic fθ lens more easily manufactured, is disclosed in Japanese Patent Laid-Open Publication No. hei 7-111501 (see FIGS. 1, 3, and 4), the entire disclosure of which is hereby incorporated by reference.
The laser scanning unit includes a first optical scanning system, which focuses light radiated from the light source and emits the focused light to a deflection point formed on a deflecting surface of light deflecting means, and a second optical scanning system, which has an fθ lens made of a single lens. Accordingly, if the thin fθ lens is made of plastic, it can be easily manufactured and mass produced at lower costs.
However, the two conventional laser scanning units have the following drawbacks. Since the former laser scanning unit obtains a magnifying power necessary for image formation in the main scanning direction using only the singlet scanning lens, the magnifying power of the scanning lens needs to be increased. Consequently, the scanning lens has a non-uniform shape, which is thick in the middle compared to its edges, thereby increasing a possibility of internal deformation during a lens forming process.
Since the latter laser scanning unit causes the focused laser beam to be incident on the fθ lens, the laser scanning unit is superior in lens forming efficiency to the former laser scanning unit. However, if an optical path from the light source to the photosensitive drum is extended, magnifying power increases such that the light source unit is required to satisfy a higher assembly precision, and assembly productivity decreases. Also, since an outer diameter of the collimating lens needs to increase, associated costs also increase, and changes in product performance due to changes in environmental conditions, including temperature increases.
An optical scanning system, which can be produced inexpensively by commonly using only one expensive deflector for four laser beams, is used for conventional tandem color printers. In this case however, both an optical path synthesizing unit for synthesizing optical paths of the four laser beams and an optical path separating unit for separating the four optical paths are necessary, thereby, increasing the entire optical path length. Accordingly, such an optical scanning system is difficult to be applied to the latter laser scanning unit.
Accordingly, a need exists for a system and method for a laser scanning unit that can be provided with reduced assembly precision requirements, and also minimize changes in performance due to changes in environmental conditions, including temperature increases.